Method for electrodeposition coating including a preimmersion deposition step



Jam 1970 I o. B. JOHNSON 3,492,213

METHOD FOR ELECTRODEPOSITION COATING INCLUDING A PREIMMERSION DEPOSITION STEP Filed June 2, 1967 '7 2 Sheet-Sheet l I 1 :5: l F I Q I [a l l l a Q I k l 4} I l I T i 1 H Q IN h 02/ a. ua/m/sozv a IN VEN TOR.

A 7'TOR/VEYS Jan. 27, 1970 o. B. JOHNSON 3,492,213

METHOD FOR ELECTRODEPOSITION COATING INCLUDING A PREIMMERSION DEPOSITION STEP Filed June 2, 1%67 2 Sheets-Sheet 2 0L nv e. Jay/v50 INVENTOR.

A 7'7'OR/VEYS United States Patent M METHOD FOR ELECTRODEPOSITION COATING INCLUDING A PREIMMERSION DEPOSITION STEP Olin B. Johnson, Livonia, Mich., assignor to Ford Motor Company, Dearborn, Mich., a corporation of Delaware Filed June 2, 1967, Ser. No. 643,128 Int. Cl. C231: 13/00; B01k 5/02 US. Cl. 204181 7 Claims ABSTRACT OF THE DISCLOSURE A method for electrodepositing a film-forming organic coating material upon an electrically conductive workpiece which comprises passing the workpiece to be coated through an aqueous stream that is in contact with a first electrode and in which the coating material is dispersed, maintaining a first dilference of electrical potential between said first electrode and the workpiece which serves as a second electrode, said dilference of electrical potential being sufficient to initiate electrodeposition of said coating material upon said workpiece, immersing the workpiece in an aqueous bath in which organic filmforming coating material is dispersed and which is in contact with a third electrode, maintaining a second dilference of electrical potential between said workpiece and said third electrode and electrodepositing additional amounts of organic film-forming coating material from said bath upon said workpiece, said second difference of electrical potential being less than said first difference 0 potential.

Background of the invention In the method of painting electrically conductive workpieces by immersing the workpiece in an aqueous dispersion of the paint and passing an electric current through the bath between the workpiece and another electrode in contact with the bath while maintaining a constant voltage, an undesirable surge of current occurs until an insulative film of the paint deposits upon the workpiece. This problem is particularly acute in batch type operations wherein the total surface area of the objects to be coated is relatively large in relation to the size of the coating bath into which they are introduced. Aside from the waste of power involved, this surge tends to heat the workpiece unnecessarily, increasing the opportunities for film rupture and otherwise deleteriously affecting the coating bath and coating process.

This problem can be controlled by employing a dual power system or by varying the potential with deposition. It has also been proposed to employ an external resistance in the coating circuit, a resistance that will blunt such surge and become relatively insignificant when a highly resistive film is built up on the workpiece.

Brief description of the invention In the method of this invention, the undesirable initial surge of current in the coating bath is avoided by passing the workpiece, prior to entry into the bath, through a stream comprising an aqueous dispersion of organic filmforming coating material that is in contact with an electrode, passing a direct electric current through such stream between the electrode and the workpiece at a potential sufficient to initiate electrodeposition of the coating material from the stream upon the workpiece and maintaining such electrodeposition until the film thus deposited upon the workpiece has sufiicient electrical resistance to materially reduce the surge of current upon immersion of the workpiece in an electrodeposition bath 3,492,213 Patented Jan. 27, 1970 from that that would result were the workpiece to enter such bath uncoated.

The partially coated workpiece proceeds immediately into the coating bath and further coating is effected in the usual manner. A higher average current density is employed in the flowcoating step than that employed in the immersion coating step.

Brief description of the drawings FIGURE 1 is a partially cut-away, schematic side view of one embodiment of apparatus suitable for carrying out the method of this invention; and

FIGURE 2 is a partially cut-away, end view of the apparatus shown in FIGURE 1.

Description of the preferred embodiments In the illustrated embodiment, tank 11 contains a coating bath 13 and serves as a negative electrode in the coating process. Tank 11 is electrically connected via conductor 15 to a negative terminal of DC. power source 17. A positive terminal of DC. power source 17 is electrically connected via conductor 19 to bus bar 21.

The articles to be coated 25 and 27 are supported by conductor hangers 29 and 31, respectively. Contact plates 33 and 35 are attached to hangers 29 and 31 respectively and provide electrical connection between hangers 29 and 31 and bus bar 21. Hangers 29 and 31 include insulators 37 and 39 which insulate the articles 25 and 27 from the grounded conveyor 41. Conveyor 41 is a conventional, electrically powered, chain driven conveyor.

Near the upstream end of tank 11, coating bath 13 is withdrawn from tank 11 via conduits 45 and 47 and passed via pumps 49 and 51, conduits 53 and 55, and flow regulation valves 57 and 59 to cathode assemblies 61 and 63 respectively from which it is discharged against the workpiece passing therebetween. Cathode assemblies 61 and 63 are respectively connected to a negative terminal of DC. power source 17 via conductors 65 and 67.

In this embodiment, bath 13 is prepared in the following manner:

An extended coupled glyceride drying oil paint binder is made by reacting in an agitator tank 8,467 parts of alkali-refined linseed oil and 2,025 parts of .maleic anhydride (heated together at 232.2 C. for about three hours until an acid value of 90 results), then cooling this intermediate to 157.2 C., adding 1,789 parts of vinyl toluene containing 48 parts of ditertiary butyl peroxide and reacting at 218.3 C. for about an hour. The resulting vinyl toluenated material is then cooled to 157.2 C. and 5,294 parts of non-heat reactive, thermoplastic, oilsoluble phenolic resin is added, the temperature raised to 232.2 C. and the mixture held one hour. The phenolic resin is a solid lump resin having softening point of C., specific gravity of 1.03-1.05 at 20 C., and has been stripped to get out excess phenol and low molecular weight materials. It is a condensation product of about equimolar quantities of para tertiary butyl phenol and formaldehyde. The electrical equivalent weight of the resulting acid resin as extended is about 1,640, and it has acid number of 65.

The material then is cooled to 933 C., and 1,140 parts are taken for forming a paint dispersion. To these 1,140 parts, 100 parts of water are added, then 13.6 parts of triethylamine, the mixture agitated for a few minutes, then 74 more parts of water and 92.5 parts diisopropanol amine added. This mixture is further reduced with 1,825 parts water and 32.5 parts diethylene triamine while agitation is continued.

To this paint dispersion there is added 50 parts of a treating mixture of mineral spirits, a light hydrocarbon liquid having A.P.I. gravity of 45-495, specific gravity at 5.6 C. of 0.78-0.80, flash point (Cleveland Open Cup) ietween 37.846 C., a negative doctor test and no acidity, .2 parts of a wetting agent (the oleic ester of sarcosine, raving a maximum of 2% free fatty acid, a specific gravity If 0.948, color on the Gardner scale of 6, and a molecular veight of 340-350). This material is compatible with the taint dispersion; no distinct hydrocarbon phase results ither at this time, even though a substantial amount of hydrocarbon (predominantly aliphatic) has been used, nor tfter further addition of the pigment grind and addition )f extra water to make the initial painting bath.

A pigment grind is made from 123 parts of vinyl-tolueiated, maleic-coupled linseed oil made in the same manner lS the resin hereinbefore shown in this example (except hat the resulting polycarboxylic acid resin is not extended vith the phenolic resin), 8.4 parts of diisopropanol amine, L7 part of an antifoam agent (a ditertiary acetylenic glycol with methyl and isopropyl substitution on the tertitry carbon atoms), 233 parts of fine kaoline clay, .155 warts of pigmentary titanium dioxide, 7.8 parts of fine ead chromate, 15.5 parts of fine red iron oxide, 16.9 )arts of carbon black, and 201 parts of water. The resultng pigment grind is then blended with the foregoing paint lispersion and treating mixture to make a concentrated )aint. The resulting paint is reduced further with water n the ratio of one part of the resulting paint per parts )f water to make an initial painting bath for electro- Jainting operations. The resulting bath has resin solids Inon-volatile matter) concentration of 7.24%. The total )f amine equivalents used in making up the initial bath s about 4.5 times the minimum amount necessary to keep his polycarboxylic acid resin, once dispersed, in anionic aolyelectrolyte condition in the bath and about 1.25 times full neutralization of the acid resin with respect to its acid number. The number of coulombs of direct current 186d to electroplate a gram of this resin on an anode at ninimum amine concentration in the bath to develop requisite polyelectrolyte characteristics for my coating process is virtually constant at 24. Specific resistance of the initial bath is about 900 ohmcentimeters.

The replacement paint solids are made by dispersing 1,140 parts of the same kind of extended polycarboxylic acid resin with 100 parts of water and 13.6 parts of triathylamine. To this is added the mineral spirits, the wetting agent, and the foregoing pigment grind, all of the same compositions and in proportions as are used to make up the original paint dispersion for the bath.

The cathode assemblies here shown are merely illustrative of a variety of cathode designs that can be employed for discharging the dispersion of coating material on the workpiece. In one preferred embodiment, the outlet means from which the dispersion is released to the workpiece is an electrically conductive grill which forms the cathode proper and divides the outflow into a plurality of streams that converge prior to or upon contacting the workpiece.

The difference of electrical potential between the cathode and the workpiece in the electrofiocoating step is advantageously in the range of about 400 to about 1,000 volts while the difference of potential in the immersion coating step is advantageously in the range of about 100 to about 350 volts, more commonly about 150 to about 250 volts. Bath resistivity is ordinarily in the range of about 500 to about 1,000 ohm-cm. and the spacing between cathode and workpiece in the electrofiocoating step is less than about 10 inches, advantageously less than about 4 inches and preferably not substantially greater than about 2 inches. In the illustrated embodiment, the spacing of cathodes 61 and 63 from the workpiece during coating is about 2 inches, the difference of potential is about 400 volts and the diameter of the streams passing between cathode and workpiece is about 1 inch.

The film deposited in the electrofiocoating step is sufficient to provide an electrical resistance to further coating that will substantially reduce the initial surge of current that occurs when an uncoated workpiece enters the coating bath. Advantageously, this film is at least about 0.05 mil in thickness and may vary upward to about 0.5 mil in thickness.

In the illustrated embodiment, the coating material employed in the electroflocoating step is taken directly from the coating bath and hence is the same as the employed in the immersion coating step. It is within the scope of this invention to employ in the electroflocoating siep a different dispersion than that employed in the immersion coating step so long as the two coating compositions are operationally compatible with each other. In the last described embodiment, a tank is employed to retain the dispersion for the electroflocoating dispersion that is separate from the main coating tank.

In this application, painting by electrodeposition is meant to include the deposition of finely ground pigment and/ or filler in the ionizable resin herein referred to as the binder, the deposition of binder without pigment and/or filler or having very little of same, but which can be tinted if desired, and the deposition of other water reducible surface coating compositions containing the binder which might be considered to be broadly analogous to enamel, varnish, or lacquer bases, and the coating material for such deposition is termed a paint. Thus, the binder, which is converted to a water-resistant film by the electrodeposition and ultimately converted to a durable film resistant to conventional service conditions by final curing, can be all or virtually all that is to be deposited to form the film, or it can be a vehicle for pigmentary and/or mineral filler material or even other resins on which it exerts the desired action for depositing the film. Suitable resins include but are not limited to those specifically listed in US. Patent 3,230,162 to A. E. Gilchrist. The preferred resins for anodic deposition have an acid number between about 30 and about 300 and an electrical equivalent weight between about 1,000 and about 20,000. The term electrical equivalent weight is employed herein to mean that amount of resin or resin mixture that will deposit per Faraday of electrical energy input. The conditions, procedures, and calculations which can be employed to determine electrical equivalent weight are set forth in detail in the aforementioned US. Patent 3,230,162.

The thickness of the film obtained in the electroflocoating step can be controlled by controlling the time the workpiece is within the stream, the difference of potential between the electrodes, the cross sectional area of the coating stream, and the distance between electrodes while electrodeposition is being carried out.

While the coating materials heretofore described herein and the films electrode osited therefrom are conventionally cured by convential curing techniques such as baking, it will be understood that it is within the scope of this invention to electrodeposit coating compositions which are also adapted for radiation polymerization, e.g. by an electron beam having an average potential in the range of about 150,000 to about 450,000 electron volts. Such resins, in addition to the ionizable carboxylic acid groups of the resins heretofore described, have about 0.5 to about 3 alpha-beta olefinic unsaturation units per 1,000 units molecular weight. Examples of such resins and methods for their preparation are described in the copending United States patent application Ser. No. 583,885, filed Oct. 3, 1966, by Arthur G. Smith and Allen H. Turner and now abandoned.

The disclosures of the aforementioned patent to Gilchrist, US. Patent 3,230,162, shall be deemed to be incorporated within this specification by reference.

The foregoing examples are solely for purposes of illustration and should not be considered as limitations upon the true scope of the invention as set forth in the appended claims.

I claim:

1. A method for electrodepositing a film-forming organic coating material upon an electrically conductive workpiece which comprises passing the workpiece to be coated through an aqueous stream that is in contact with a first electrode and in which said coating material is dispersed, maintaining a first dilference of electrical potential between said first electrode and said workpiece, said workpiece serving as a second electrode, said difference of electrical potential being sufficient to initiate electrodeposition of said coating material upon said work piece, immersing said workpiece in an aqueous bath in which an organic film-forming coating material is dispersed and which is in contact with a third electrode, and maintaining a second difference of electrical potential between said workpiece and said third electrode causing a direct current of electrical energy to pass through said bath between said third electrode and said workpiece electrodepositing upon said workpiece from said bath additional amounts of organic film-forming coating material, said ditference of electrical potential being less than said first difference of potential.

2. The method of claim 1 wherein said first difference of electrical potential is in the range of about 400 to about 1,000 volts and said second difference of electrical potential is in the range of about 100 to about 350 volts.

3. The method of claim 1 where said first electrode is spaced a distance of less than about inches from said workpiece when said workpiece is passing through said stream, said first difference of potential is sufiicient to provide at the surface of the workpiece while said workpiece is passing through said stream an average current density that is at least twice as great as the maximum tolerable average current density for electrodepositing a continuous film of said coating material of at least about 0.5 mil thickness without film-rupture when said workpiece is immersed in the same dispersion and moved therethrough at a rate of less than about 40 feet per minute, and said second diiference of potential is sufiicient to provide at the surface of the immersed workpiece an average current density during electrodeposition that is sufiicient to effect electrodeposition of said coating material from said bath upon said workpiece and is less than said maximum tolerable average current density.

4. The method of claim 1 wherein a film of organic film-forming material having a thickness in the range of about 0.05 to about 0.5 mil is electrodepositcd upon said workpiece by said first difference of potential prior to immersion of said workpiece in said bath.

5. The method of claim 1 wherein said dispersion of organic coating material is an aqueous dispersion of a synthetic, polycarboxylic acid resin at least partially neutralized by a water soluble amine.

6. The method of claim 1 wherein said stream is formed by continuously withdrawing an undivided portion of said bath, passed as a continuous stream in simultaneous contact with said first electrode and said workpiece, and continuously returned to said bath.

7. The method of painting an electrically conductive workpiece which comprises dispersing in an aqueous bath a water soluble amine and a paint having as the predominant fraction of the film-forming binder thereof a synthetic, polycarboxylic acid resin, removing a continuous stream of the resulting dispersion from said bath, passing said stream in contact with a first electrode, passing said workpiece through said stream, maintaining a first diiference of electrical potential in the range of about 400 to about 1,000 volts between said first electrode and said workpiece until a. film of said paint having an average thickness of at least about 0.05 milis electrodeposited upon said workpiece, said workpiece serving as a second electrode and being spaced from said first electrode a distance not substantially in excess of about 2 inches when said workpiece is passed through said stream, immersing said workpiece in an aqueous bath in which an organic film-forming coating material is dispersed and which is in contact with a third electrode, and maintaining a second difference of electrical potential between said workpiece and said third electrode in the range of about to about 225 volts and electrodepositing upon said workpiece from said bath additional amounts of organic film-forming coating material.

References Cited UNITED STATES PATENTS 1,590,601 6/1926 Taylor 204-181 3,325,390 6/1967 Burnside et al. 20418l 3,399,126 8/1968 Turner 2O4l81 3,399,128 8/1968 Brewer et al 204181 3,420,762 1/1969 Shaw et al. 204181 JOHN H. MACK, Primary Examiner E. ZAGARELLA, JR., Assistant Examiner US. Cl. X.R. 204300 

